Footwear is divided generally into two parts, an upper and a sole. The upper is the portion of the footwear designed to comfortably enclose the foot, while the sole is the portion of the footwear designed to provide traction, protection, cushioning, and a durable wear surface. Typically, the sole includes several layers, including a resilient, shock absorbent material as a midsole and a ground engaging durable material as an outsole.
Known midsoles are typically made of conventional foam materials, such as ethylene vinyl acetate (EVA) or polyurethane. These materials compress resiliently under an applied load, such as forces generated by athletes, to provide cushioning to the athlete's feet and legs. Conventional foam materials are resiliently compressible, in part, due to the inclusion of foam having open and closed cells defining an inner volume that is substantially displaced by gas. In other words, the foam includes bubbles within the material which give the foam its compressible and resilient features. Conventional foam materials, however, have certain drawbacks. Most notably, the foam materials deteriorate by compaction after repeated compression caused by extended use of the footwear. The deterioration is the result of the collapse of the cell structure within the foam, resulting in decreased compressibility of the foam and thus decreased cushioning of the midsole. A mostly successful solution to the problems associated with conventional foam materials has been the use and placement of gas-filled cushioning devices or bladders within the midsole. The bladders are inflatable inserts made of elastomeric materials that are resiliently compressible to provide cushioning to the wearer of the footwear.
There are several known bladders of varying construction, which have been used in the footwear industry. For instance, U.S. Pat. No. 6,119,371 to Goodwin, the disclosure of which is incorporated by reference, discloses a bladder formed of an outer enclosing member and an inner member. The outer enclosing member is formed of a thermoplastic polyurethane (TPU) film consisting of two layers of material. The TPU film forms the shell shape of the bladder and defines a sealed chamber. The inner member is located within the sealed chamber and is formed of spaced apart fabric layers connected together by a plurality of connecting yarns or drop threads. The sealed chamber contains a pressurized fluid to place the plurality of connecting yarns under tension. The resulting structure provides enhanced cushioning to the wearer of the footwear.
Other types of bladders are disclosed in U.S. Pat. Nos. 5,713,141 and 5,952,065 to Mitchell et al., U.S. Pat. No. 5,353,459 to Potter et al., and U.S. Pat. Nos. 4,506,460 and 4,219,945 to Rudy, the disclosures of which are incorporated by reference. These bladders are known generally as permanently filled bladders and may be manufactured by various techniques. For example, some bladders are manufactured by the prior art two-film technique in which two sheets of elastomeric film are welded together along their periphery to define an enclosed chamber. Other bladders are manufactured by the prior art blow molding technique in which a liquefied elastomeric material is placed in a mold having the desired shape of the bladder. Pressurized air is then injected into the mold to force the elastomeric material against the inner surfaces of the mold and cause the material to harden to the desired bladder shape. With either technique, the formed bladder is pressurized with a gas or liquid and sealed to create an inflated resilient bladder that provides added footwear cushioning. It will be recognized that some of these known bladders are made of multiple chambers in fluid communication with each other. Those of skill in the art will recognize that other footwear bladders exist, the details and construction of which, however, need not be discussed except to note that these bladders may be used with the present invention.
A drawback with respect to existing bladders is over flexing of the TPU film of the outer chamber. The over flexing is a result of abnormal wear of the shoe caused by abnormal loading placed on the bladder from athletes who, for example, have a tendency to pronate or supinate while running. Over time, this over flexing leads to film cracking and fatigue and ultimately bladder failure due to subsequent pressure loss in the chamber. The failures of the bladder are most prevalent in the vertical side walls of the bladder, particularly in exposed bladders—bladders not enveloped in a foam material, but visible to the outside.
In an effort to overcome the failure of existing bladders, conventional foam materials were placed around the bladder. While this effort has been somewhat successful, problems still exist with respect to the use of foam materials. As stated above, the cell structure of the foam material has a tendency to break down and deteriorate over time and use. Upon the break-down of the foam material, the bladders lose their structural support provided by the foam material and experience over flexing of the TPU film. Moreover, with the use of a foam material enclosing the bladder, the material will mask the pure cushioning response of the bladder, thus changing the perception of the footwear.